Bacterial cytidine–phosphate–guanosine (CpG‐DNA) activates antigen‐presenting cells (APC) and drives T helper 1 (Th1)‐polarized immune responses in the mouse. Claims have been made that CpG‐DNA costimulates murine T cells. We examined the direct and indirect effects of CpG‐oligodeoxynucleotides (CpG‐ODN) on human T‐cell activation. CpG‐ODN failed to costimulate purified human T cells activated with α‐CD3 or α‐T‐cell receptor (TCR)αβ antibodies. In contrast, CpG‐ODN sequence‐specifically caused increased expression of CD69 on CD4 and CD8 T cells when peripheral blood mononuclear cells (PBMC) were stimulated via α‐CD3. CpG‐ODN and α‐CD3 stimulation synergized to induce interferon‐γ (IFN‐γ) in T cells and natural killer (NK) cells, as shown by intracellular fluorescence‐activated cell sorter (FACS) staining. These effects of CpG‐ODN on human T cells were caused by the release of IFN type I (IFN‐I) and interleukin‐12 (IL‐12) from PBMC. Enhancement of CD69 expression on α‐CD3‐triggered T cells could be reproduced in a coculture transwell system of purified T cells and PBMC, was inhibited by neutralizing antibodies to IFN‐I and could be mimicked by adding exogenous IFN‐I. Furthermore, neutralization of either IFN‐I or IL‐12 diminished, and in combination abolished, IFN‐γ production. These findings show that CpG‐ODN potentiate TCR‐triggered activation of human T cells in an APC‐dependent manner.